feature: Add Prism DI extensions

[glennawatson]

Change the locator to be accessible in a method

Update dependencies where appropriate

Fixes #358

[codecov]

Codecov Report

Merging #431 into master will increase coverage by 2.12%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
+ Coverage   64.65%   66.77%   +2.12%     
==========================================
  Files          70       70              
  Lines        4111     3943     -168     
  Branches      419      394      -25     
==========================================
- Hits         2658     2633      -25     
+ Misses       1377     1237     -140     
+ Partials       76       73       -3

Impacted Files	Coverage Δ
src/Splat/MemoizingMRUCache.cs	56.32% <0%> (ø)	⬆️
src/Splat/AssemblyFinder.cs	0% <0%> (ø)	⬆️
src/Splat/ServiceLocation/Locator.cs	76.92% <100%> (ø)	⬆️
.../Splat/ServiceLocation/DependencyResolverMixins.cs	37.5% <100%> (ø)	⬆️
.../Splat/ServiceLocation/ModernDependencyResolver.cs	50.74% <66.66%> (-1.5%)	⬇️
src/Splat.Drawing/Colors/SplatColor.KnownColors.cs	0% <0%> (-10.93%)	⬇️
src/Splat.NLog/NLogLogger.cs	73.74% <0%> (-0.84%)	⬇️
src/Splat.Drawing/Colors/SplatColor.cs	24.6% <0%> (+11.4%)	⬆️

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